Add a module for the prebuilt libunwind.

We've thus far been missing this in r13. Apparently ndk-build doesn't
actually consider non-existent dependencies to be an error (or even a
warning), so the lack of this module hasn't caused any build time
issues.

I'm adding this to libc++'s makefile rather than libunwind because we
don't want to expose libunwind as an NDK API.

Test: ndk/run_tests.py --filter libcxx-unwind-static
Bug: http://b.android.com/223708
Change-Id: I58e8c6787e3e282556bb3a4f0267998fa5eeee70
(cherry picked from commit d6db8480984da8563571632476ccde4a69861d1e)
diff --git a/Android.mk b/Android.mk
index 0d3e68d..3b77543 100644
--- a/Android.mk
+++ b/Android.mk
@@ -98,6 +98,15 @@
 LOCAL_EXPORT_LDFLAGS := $(libcxx_export_ldflags)
 include $(PREBUILT_SHARED_LIBRARY)
 
+ifneq (,$(filter armeabi%,$(TARGET_ARCH_ABI)))
+# We define this module here rather than in a separate cxx-stl/libunwind because
+# we don't actually want to make the API available (yet).
+include $(CLEAR_VARS)
+LOCAL_MODULE := libunwind
+LOCAL_SRC_FILES := libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE)$(TARGET_LIB_EXTENSION)
+include $(PREBUILT_STATIC_LIBRARY)
+endif
+
 $(call import-module, cxx-stl/llvm-libc++abi)
 
 else